1. Field of the Invention
The present invention relates to an arrangement for severing or cutting a tension member of a soil anchor composed of ferromagnetic material at a predetermined location thereof. The tension member is provided with a predetermined breaking point by reducing its tensile strength. The arrangement includes a coil which can be placed or mounted in a bore hole together with the tension member and surrounds the tension member in a tubular manner at the intended severing point. Electric current can be applied to the coil for producing a predetermined breaking point by reducing the tensile strength of the tension member as a result of thermal influence due to induction.
2. Description of the Related Art
A soil anchor includes a tension member which is introduced into a bore hole opening and a bonding action is obtained with the bore hole wall and, thus, with the surrounding soil, by pressing in hardening material, such as, cement, mortar or the like. This produces a pressing body which is connected in a frictionally engaging manner with the structural component to be anchored through the remaining part of the tension member which extends to the bore hole opening. The tension member may be composed of a single element or of several elements which, in turn, may be composed of steel rods, steel wires, steel strands or even steel pipes. The length of the tension member over which the tension member is embedded in the pressing body is called the anchoring length, and the remaining length of the tension member, which for the purposes of prestressing is freely extendable, is called the free steel length.
Soil anchors may be used for the permanent anchoring of structures in the soil. However, they can also be used temporarily, such as, for the rearward anchoring of a wall in an excavation. If a temporarily mounted soil anchor extends into a neighboring piece of land, it must usually be removed after the construction work during which it was employed has ended.
For removing a soil anchor, a possibility for severing the tension member is provided usually at the transition between the anchoring length and the free steel length of the tension member, so that the free part of the tension member can be pulled out of the bore hole and possibly recovered. The pressing body itself, which rarely has a length greater than about four to eight meters, can usually be easily removed if excavation work in the neighboring piece of land is carried out over the surface area thereof, for example, by means of bulldozers.
Of the various possibilities for severing the tension member of a permanent soil anchor, the use of heat for reducing the strength of the steel tension member is most important because the means required for producing the heat can be mounted together with the tension member without significantly enlarging the bore hole diameter and can be kept operational over a longer period of time. In addition, if for severing the tension member a predetermined breaking point is provided by reducing its strength by means of thermal influence, the tension member can be utilized with its full cross-section during the entire duration of its use.
For producing the heat necessary for reducing the tensile strength of the tension member, it is known in the art to heat the tension member in the area of the predetermined breaking point by means of an exothermic reaction, for example, an aluminothermic mixture (FR 22 74 740). In order to release the exothermic reaction, an ignition system is required which is difficult to maintain capable of ignition in the mounted state of the anchor, wherein the mounted state may extend over a long period of time.
It is also known in the art to use electrical energy for producing heat. This can be effected by means of an electric heating element, for example, in the form of a heating coil surrounding the tension member (DE 24 28 729 C3), or also by means of a coil which surrounds the tension member at the severing point and which is supplied with electric current in order to produce heat through induction in the tension member or tension members (CH 603 919). It has been found that, even when thermal insulation layers were arranged, it was not possible to achieve a temperature sufficient for reliably severing the tension member because a large portion of the supplied heat is lost through thermal conduction through the tension member itself.